1. Field of the Invention
The present invention relates to a water purification material capable of selectively adsorbing dissolved components, such as phosphate ions, contained in water such as river water, lake water, marsh water, sewage and industrial wastewater.
2. Background Art
Recently, the rapid globalization of economic activities has brought serious worldwide problems of water pollution and environmental pollution. At the same time, production activities in the global scale have led to exhaustion of resources and, as a result, not a few elements are getting regarded as rare elements.
Phosphorus is one of such rare elements. In these days, phosphorus ore has been becoming scarce in the global scale. Accordingly, phosphorus has been already considered as a rare element in recent years. On the other hand, as one of the measures against eutrophication problems of closed waters such as lakes, marshes and bay waters, phosphorus emission standards are strictly regulated. For removing phosphorus substances from the waters, it is widely known that coagulants such as calcium compounds are added so as to coagulate and precipitate the substances. However, since phosphates in water are generally in the form of poorly sedimentary floating matter, it is necessary to form floc so as to rapidly precipitate the phosphates. Consequently, a great deal of sludge is produced.
In order to treat the sludge in a large amount, it is inevitably required to scale up the treatment facilities. Accordingly, this costs a lot of money. Further, the coagulants make the floc contain various kinds of ionic components, and it is costly to separate them from the sludge. Because of these reasons, the sludge is very often not reused but disposed of as industrial waste at great cost. This is a problem.
As described above, in the case where phosphorus in water is removed according to conventional methods, there are various inefficient problems in the process of coagulation and precipitation by adding calcium salts. For example, it takes a long time to treat the phosphorus substances in water and moreover it is necessary to scale up the facilities and to dispose of the sludge.
In view of the above problems, a lot of new materials for water purification have been proposed recently. For example, as for removal of phosphorus substances, an adsorbent having a hydrotalcite structure is proposed as a high performance phosphorus remover (e.g., in Journal of Water and Environment vol. 22, No. 11, 875-881(1999), [in Japanese]). Hydrotalcite is a kind of layered inorganic mineral compounds, and negative ions are contained among the layers in the structure thereof. It is reported that, since the negative ions among the layers are exchanged with phosphate ions in water to remove them, the adsorbent can exhibit high phosphorus removability.
However, even the hydrotalcite cannot selectively ion-exchange only the phosphate ions in water. If carbonate ions or sulfate ions, for example, are contained in the water, they are ion-exchanged prior to the phosphate ions. This means that there is room for improvement in the selectivity to phosphate ions.
From the conventional viewpoint, namely, from the viewpoint of only the water pollution and environmental pollution, it is enough to remove harmful components from water. However, in consideration of the aforementioned problem of the fact that valuable resources are getting exhausted in the global scale, it is very important in the coming society to take into account the viewpoint of recovery and reuse of the resources. Accordingly, it is very important for the coagulants and adsorbents to be excellent in the selectivity as well as in the abilities of coagulation and adsorption, respectively. Particularly in regard to phosphorus, although a large amount of substances thereof is contained not only in household wastewater but also in industrial wastewater drained from food factories, drug factories and the like, there is the serious problem that phosphorus as a resource is becoming exhausted. It is, therefore, desired to provide an adsorbent exhibiting such high performance and selectivity as are not attained by conventional adsorbents or coagulants.
Hitherto, there have been presented various proposals concerning removers of water-dissolved components, particularly, of dissolved phosphorus, which is nowadays a resource feared to be exhausted. Most of the proposed removers are coagulants or adsorbents, but they catch not only phosphorus but also many other elements and hence have a problem of insufficient selectivity to phosphorus. Since having poor selectivity to phosphorus, those coagulants or adsorbents are to be disposed of as waste after they are used. This is a serious problem from the viewpoint that phosphorus should be efficiently recovered and reused. Further, it is another problem that known coagulants form floc so as to promote precipitation of the floating matter and consequently generate a large amount of sludge.